Positive inotropic agents (i.e., agents which increase the contractility of cardiac muscle in a dose dependent manner) find use, inter alia, in the treatment of congestive heart failure and as vasodilators. Representative of the three classes of positive inotropic agents are the Na.sup.+ /K.sup.+ ATPase inhibitor digitalis, the .beta.-adrenergic agonists dobutamine and dopamine, and the phosphodiesterase inhibitor amrinone.
Each of these classes of positive inotropic agents suffers from significant limitations. Digitalis displays a weak positive inotropic effect with narrow a therapeutic index, many adverse side effects, and undesirable interactions with other cardiac drugs. Dobutamine and dopamine cause desensitization of the .beta.-adrenergic receptor-mediated positive inotropic response, are arrhythmogenic, and can only be administered intravenously. Orally active .beta.-adrenergic agonists are only effective for a short period of time and lose efficacy due to desensitization. Phosphodiesterase inhibitors, such as milrinone, are potentially arrhythmogenic and have increased mortality relative to digitalis.
ATP is known to cause an inotropic effect in the heart, which is thought to be mediated by the P2 purinergic receptor (P2PR). To date there has been no detailed characterization of the specific P2PR involved, and no suitable cell model exists for the characterization of P2PR.
Consequently, there is a need in the art for positive inotropic agents which overcome the disadvantages associated with known agents, as well as a need for further information on the mechanisms and receptors associated with cardiac muscle contractility.